Ion sources for ion implantation apparatus

ABSTRACT

The invention relates to improving the efficiency of ion flow from an ion source, by reducing heat loss from the source both in the ion chamber of the ion source and its constituent parts (e.g. the electron source). This is achieved by lining the interior of the ion chamber and/or the exterior with heat reflective and/or heat insulating material and by formation of an indirectly heated cathode tube such that heat transfer along the tube and away from the ion chamber is restricted by the formation of slits in the tube. Efficiency of the ion source is further enhanced by impregnating and/or coating the front plate of the ion chamber with a material which comprises an element or compound thereof, the ions of which element are the same specie as those to be implanted into the substrate from the source thereof.

BACKGROUND OF THE INVENTION Field of the Invention

This invention is concerned with improvements in or relating to ionsources particularly though not exclusively for use in or with ionimplantation apparatus.

A typical ion source of the type with which the present invention isconcerned comprises an ionisation, or arc, chamber (also sometimesreferred to as a plasma chamber) into which source material can beintroduced to generate a plasma in which the source material, or acomponent thereof, is ionised and the ions drawn off through an outletfrom the chamber. One typical ionisation chamber has a pair of cathodesarranged in opposed relationship, one referred to as a cathode and theother as a counter cathode. In use, the cathode is heated and emitselectrons. The counter cathode then repels the electrons so that theyare entrained between the two electrodes. The chamber is typicallywithin a magnetic field which causes the electrons to move in fixedpaths (e.g. spiral) between the two electrodes. Ionisation of the sourcematerial is achieved by the application of energy to the source materialin a number of different ways, e.g., by applying an arc potentialbetween the cathode and the chamber body or by use of r.f. energy.Interaction of the electrons with the source material introduced intothe chamber causes ionisation of the source material. An extractionelectrode which is negatively biased relative to the chamber itself ismounted at the outlet from the chamber, typically outside the chamber.The negatively biased extraction electrode draws the ions through theoutlet and the ions then pass through an aperture in the extractionelectrode to the ion implantation apparatus proper.

The present invention has as its principal objective to improve theoutput of a required ion species of existing ion sources. The efficiencyof an ion source is affected by, for example, the efficiency of theionisation chamber itself, the elements within the chamber such as theelectrodes and the output from the chamber, i.e. the quantity of ionsemitted from the chamber.

SUMMARY OF THE INVENTION

We have found that the efficiency of the chamber itself can besignificantly improved by retaining heat within the chamber body andthus within the chamber cavity. Accordingly, the present inventionprovides an ion source comprising an ionisation chamber within which aplasma can be generated, the chamber having an outlet through which ionscan exit from the ionisation chamber, electrodes in the ionisationchamber for establishing and maintaining a plasma within the chamberwhen a power supply is provided thereto, and a heat shield enclosing atleast a part of the ionisation chamber to retain heat within the chamberwhen the ion source is functioning.

There are various ways in which heat shields can be provided for theionisation chamber. A heat shield can be provided by a simple coating ofmaterial on the exterior surfaces of the chamber, or by a plurality ofsuch coatings. If one or more coatings is/are employed, then the singlecoating, or the innermost of a plurality (i.e. that applied directly tothe exterior of the chamber) preferably has/have heat reflectiveproperties, such that heat generated by the plasma within the chamberand conducted through the body of the chamber is retained in the chamberthereby causing the temperature of the chamber body to increase and thusalso of the chamber interior. As a preferred alternative, or in additionto coating the exterior of the chamber, a plurality of heat shieldmembers may at least partially surround the chamber so that the chamberis effectively clad in preformed heat-reflecting, and possiblyheat-insulating material, most practically in the form of plates orpanels of such material, either provided as single plates or panelswhich can be mounted on or around the chamber to enclose at least a partof the chamber, or by two or more plates or panels of heat-reflectivematerial, such as stainless steel, with the plates or panels arranged inparallel, spaced apart, relationship to one another to define a gapbetween adjacent plates or panels (hereinafter referred to as ‘plates’only).

Where the chamber is typically of cuboid shape, the plates would berectangular to form an exterior heat shield for the chamber, with theplates extending in their spaced apart, parallel, relationshipco-extensively with an adjacent wall of the chamber, the platesextending along one wall being coupled to plates extending along eachadjacent wall by coupling members adjacent each corner edge of thechamber. The coupling members may each, in a preferred embodiment, beprovided with wing portions defining elongate spaced wall portions,between adjacent ones of which a slot is defined. Each slot is of awidth and length such that an edge portion of a plate can be locatedtherein whereby, from one end of the plates where they are mounted inthe slots of a wing portion of a coupling member, plates can extend inspaced parallel relationship to the next adjacent coupling member, whilebeing also spaced from the adjacent wall of the chamber.

As a further alternative, the heat shield may be in the form of a hollowmultipart body into which the ionisation chamber can be fitted. Such astructure can be assembled in the same manner as above described. As ayet further alternative, such a hollow multipart body may be ofsufficient size to accommodate an insulating chamber having heatinsulating and/or reflecting plates already provided thereon.

The coatings and/or external plates where these comprise reflectivematerial may be formed to provide a reflective surface of metal, e.g.tungsten, molybdenum, nickel, tantalum, or of compounds thereof or ofmetallic alloys and/or compositions, but, as stated above, may be e.g.stainless steel provided that this does not affect beam purity.

Where a liner is contemplated for the chamber interior in order toenhance heat retention, the material selected therefor must be suchthat, when the chamber is in use, the material does not detrimentallycontaminate the ion flow from the chamber. Suitable materials would betungsten, molybdenum, graphite and silicon carbide and other materialsprovided that the melting points thereof are above that attained withinthe chamber when in use.

The present invention also provides an ion source comprising anionisation chamber within which a plasma can be generated, an outletthrough which ions can exit the chamber, electrode means in theionisation chamber for establishing and maintaining a plasma within thechamber when a power supply is provided to the electrode means, and ascreen provided on or adjacent an interior wall of the ionisationchamber, electrically insulated therefrom, the screen being heatedduring operation of the ion source to assist ejection of ions therefromthrough said outlet and/or the breakdown of molecular species within thechamber. Heating may be either by passing an electrical current throughit, or applying a bias to it so that it is heated by bombardment by theplasma constituents to help with the breakdown of molecular specieswithin the source. Biasing the plate with respect to the chamber alsoassists the ejection of ions therefrom through said outlet.

It is preferred that the screen of an ion source according to thepresent invention is formed of metallic materials such as tungsten,molybdenum or compounds or alloys thereof. Alternatively, the materialof the screen may be selected from materials which can contribute ionsof the same species as are derivable from source material introducedinto the chamber. The screen may be formed as a laminate of a layer ofmaterial capable of providing ions and a layer of low thermalconductivity material, with the latter facing the adjacent wall of thechamber.

The screen may be provided by a rectangular plate with the plate mountedon support pins which extend through the wall of the ionisation chamber.The plate may be supported on said support pins in spaced relationshipto the wall of the ionisation chamber. When the source material issupplied in gaseous form and the chamber has an inlet port forintroduction of such gaseous material, it is preferred that the screenoverlies said inlet port to cause reactant gas or gases to dispersearound it when flowing into the ionisation chamber.

One of said support pins is electrically conductive for coupling thescreen to a variable bias supply for biasing the screen with respect tothe chamber.

The screen within the chamber is maintained at a potential slightly morepositive, or in some cases negative, than that of the chamber itself,which may be at ground or some other selected potential. The screen isdesigned so that, due to bombardment by constituents of a plasma in thechamber, it becomes heated.

It has been observed that, under such conditions, using a screen in theform of a tungsten plate (and where boron ions are to be produced from asource such as BF₃), there is a significant increase in B ion beamcurrents when the screen is biased positively relative to the chamberitself. A similar increase is observed for Ar ion beam currents when agaseous Ar supply is used as the ion source feed material.

It is believed that the effect of increased ion beam current is due tomodification of the plasma due to one or more of the electrostaticfields created in front of the screen, modification of the electronenergy distribution within the plasma and/or the heated surface of thescreen increasing the proportion of for example B ions as opposed to BFions and BF₂ ions resulting from the initial BF₃ source material.

To enhance the efficiency of an ion source still further, it isimportant also to consider the efficiency of the cathode unit.

Accordingly, the present invention further provides in another aspect acathode unit for use in an ion source of an ion implanter, wherein theion source comprises an ionisation chamber incorporating said cathodeunit, and an outlet through which ions can exit from the chamber, thecathode unit comprising a cathode tube arranged to extend through one ofsaid opposed wall portions, a cathode button mounted at the end of thecathode tube within the chamber and a heater element positioned in thecathode tube for connection with a power supply negatively biasing theheating element (e.g. a filament) relative to the cathode button to emitelectrons thereby to heat the cathode button to cause the cathode buttonto emit electrons into the ionisation chamber, the tube having a lengthand having at least one slit with a component transverse to the lengthsuch as to restrict heat conduction along the cathode tube.

In a preferred embodiment of this aspect of the present invention, acathode unit may comprise two or more slits spaced along the length ofthe cathode tube, and/or two or more slits spaced around the cathodetube. Preferably, the slits are uniformly spaced around the tube. In oneform of cathode unit according to the invention, each slit lies in arespective plane which is normal to the direction of the length of thecathode tube. A plurality of such slits can be arranged in adiscontinuous ring around the tube, and ideally the ring is formed bybetween two and eight evenly-spaced slits, which may be of equal orsubstantially equal length, though it is not essential that this is so.Between adjacent slits of a ring of slits a neck portion is defined; thenarrower the neck portion, the less heat can be conducted along thetube.

The provision of slits as compared with the prior art, in which muchlarger cut-outs were formed in the tube, provides the benefits andadvantages of reducing gas loss and gas flow from the arc chamber andreduction of heat loss from the cathode.

In one embodiment of a cathode tube according to the present invention,the slits are formed in the cathode tube sufficiently closely and insuch spaced relationship to one another as to define a tubular mesh.

When the ion source in which the cathode unit is mounted is in use andthe cathode button is heated by bombardment of the cathode button byelectrons from the filament, provision of the one or more slits formedin the cathode tube restricts heat conduction along the tube away fromthe cathode button. Consequently, the cathode button retains heat whichwould otherwise be dissipated along the tube. We have found that thishas the effect of increasing ion beam current emanating from theionisation chamber as compared with known cathode units of the samegeneric indirectly-heated cathode type. For example, an increase ofabout 15% has been measured for a low energy boron ion current. Inconsequence, it is possible, using a cathode unit according to thepresent invention, to operate an ion source more efficiently, withslightly less gas flow into the chamber to achieve the same ion currentas before. Greater efficiency also reduces the rate at which insulatorsin the chamber (for mounting the electrodes for example) become coatedwith material condensing onto these elements as the time required toproduce the same beam current as before is shortened. The cathode tubeand the cathode button are typically formed of tungsten or molybdenum.

The cathode button may be shaped to provide a press fit with and at theend of the cathode tube, or may be integrally formed as a single onepiece unit with the cathode tube. Where the tube and the cap are formedseparately, the cap can be secured to the end of the tube by welds.

A cathode tube according to the present invention may be of any desiredcross section but will preferably be of circular cross-section, mostpreferably of uniform circular cross-section.

An aperture may be formed in the body of the cathode tube for feedingand locating a filament and its attendant connecting wiring into thetube.

In another aspect, the present invention further provides a front faceelectrode for use as the front face of an ion source arc chamber, thefront face electrode having an aperture through which ions can beextracted from the ionisation chamber when containing atoms of apredetermined species, and the electrode having an exposed surfacecontaining further atoms of said predetermined species. Theimpregnated/coated material may also be placed in other parts of the arcchamber (e.g. as the shield previously described or as a side or endliner).

The front face electrode may be formed of a material, comprising, forexample, graphite, impregnated with said atoms. By way of example, suchatoms may comprise atoms of an element selected from the groupconsisting of boron or boron nitride, where boron ions are createdwithin the chamber using a source such as BF₃. The front face electrodeis generally of planar form and uniformly impregnated with saidmaterial.

The present invention further provides a front plate for an ionisationchamber for use in extracting boron ions from the ionisation chamber forsupply to an ion implantation apparatus, the front plate being mountableto provide an outlet of the ionisation chamber through which boron ionsare directed from the ionisation chamber, and being formed of a materialcomprising graphite or other material (e.g. tungsten) impregnated with amaterial selected from the group consisting of boron or boron nitride.

The present invention further provides a front face plate for use as thefront face plate of an ion source arc chamber, the front face platehaving an aperture through which ions can be extracted from theionisation chamber when containing atoms of a predetermined species, andthe plate having an exposed surface coated with a material whichcomprises an element or compound thereof, ions of which element orcompound are of the same predetermined species as those in theionisation chamber. The plate may again be formed of a materialcomprising graphite coated with a material selected from the groupconsisting of boron or boron nitride, where boron ions are createdwithin the chamber using a source such as BF₃. Both faces of the platemay be coated with said material whereby the plate can be reversed whenthe coating on said one face is exhausted.

We have found that, with a front face plate, which may also be used asan extraction electrode, according to the present invention, wherein anelectrode made of graphite and coated with boron carbide was used,a >10% increase in a 10 keV boron ion beam current was observed. It isbelieved that this is due to the plate being eroded during use of thechamber so that boron will enter the chamber and be ionised and theboron ions then extracted as part of the ion beam.

Although reference has only been made to use of a front face plateaccording to the invention being used to enhance boron ion creation andextraction, it will be readily appreciated that it is equally possible,within the scope of the present invention, to provide such plates, orextraction electrodes, coated or impregnated with other elements orcompounds, according to the ions to be created in the ionisationchamber.

There now follows a detailed description of various aspects of thepresent invention, illustrated by way of example with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a generally diagrammatic view of an ion implanter apparatusaccording to the present invention incorporating an ion source accordingto the present invention;

FIG. 2 is a plan view of an ion source according to the presentinvention;

FIG. 3 is an end sectional view taken on the line B—B of the ionisationchamber shown in FIG. 2;

FIG. 4 is a sectional view, taken on the line A—A of the ionisationchamber shown in FIG. 2;

FIGS. 5A to 5C are views of parts of an electron source cathode unitaccording to the invention for an ion source according to the presentinvention;

FIG. 6 is a perspective view of an ion source according to the presentinvention; and

FIG. 7 is an enlarged schematic perspective view of a portion of FIG. 6enclosed by the circle A therein.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an implantation apparatus, including an ion sourceaccording to the present invention, is illustrated schematicallytherein. In the apparatus, ions for implanting into a substrate aregenerated in an ion source indicated generally by the block 10 anddescribed below with reference to FIGS. 2 to 4. The ion source 10comprises an ionisation chamber mounted on a housing 11 by means of aninsulating bushing 12, so that the ion source can be biased relative tothe housing to generate the required extraction potential to extractions from the source and accelerate them to the required transportenergy of the ion beam. Ions are extracted from the source through anoutlet in the form of a slit 13 formed in a wall of the ionisationchamber and accelerated to the required transport energy by thepotential difference between the slit and one or more extractionelectrodes illustrated generally at 14.

The extraction electrodes illustrated generally at 14 in FIG. 1 aretypically provided externally of the ionisation chamber 10. (Similarelectrode arrangements are disclosed in our U.S. Pat. Nos. 5,883,391,5,920,076 and 5,977,552 which are hereby incorporated herein byreference.)

In U.S. Pat. No. 5,920,076, there is disclosed a known extractionelectrode structure which is typical of a structure that can be usedwith an ion source according to the present invention. In thatstructure, a pair of electrodes is spaced from the ion source, asrepresented generally at 14 a, 14 b in FIG. 1 of the accompanyingdrawings. The electrodes are also spaced and insulated from each other.Opposed apertures are formed in the electrodes in line with the slit 13in the chamber of the ion source.

In generating an ion beam 7, the ion source is voltage biased positive(for the extraction of positive ions) relative to the extractionelectrodes, and ions are extracted from the ion source, acceleratetowards the electrodes and pass through the apertures therein. Theelectrode furthest from the ion source 10 is maintained at groundpotential. An electrode is generally biased negative with respect toground (in this embodiment the one closest to the ion source) and servesto prevent electrons which are present in the space forward of theextraction electrode pair, and are required to neutralize the ion beam,from sweeping back to the ion source.

The ion beam emanating from the ion source under the influence of theextraction electrode structure is tuned to the required energy and beamcurrent by adjusting the voltage of the electrode 14 b and the ionsource and/or adjusting the size of the gap between the ion source andthe electrode structure. The position of the electrodes relative to theion source can also be adjusted to match the electrodes optically to theion source.

Ions extracted from the ion source through the extraction electrodes 14a, 14 b then pass from the ion source housing 11 into the flight tube 15of an analysing magnet 16. In the analysing magnet 16, the ions in thebeam 7 from the source travel through a region of strong magnetic fieldcausing the ions to adopt flight paths having radii of curvaturedependent on the mass/charge ratio of the individual ions.

Ions of a predetermined range of mass/charge ratios travel through theanalysing magnet in curves to emerge substantially at right-angles tothe original beam path, into a mass selecting region 17 of theimplantation apparatus, the mass selecting region containing one or moreslits to define precisely the mass/charge ratio selected by theapparatus for implanting into a substrate.

In the form of ion implantation apparatus illustrated, the ions may beextracted from the ion source 10 and accelerated to energies of about 10keV-60 keV, and preferably 40 keV to 50 keV, though the energies may beeither lower or higher than the range limits provided as example. Theions are retained at this energy throughout their passage through theanalysing magnet 16 and the mass selection region 17. For this purpose,the flight tube 15 of the analysing magnet, the housing 18 of the massselection region and the housing 11 are maintained at uniform potential.The ion source 10 is biased at the extraction potential, e.g. 40 keV or50 keV relative to this flight tube ground potential, to generate therequired extraction bias.

In a practical implantation apparatus, implantation energies of up to200 keV or more may be required, so that it is necessary to acceleratethe ions (still at a maximum 60 keV leaving the mass selection region17) to the higher required implantation energy. For this purpose,housing 19, containing the semiconductor wafer to be implanted isinsulated from the housing 18 by means of insulating bushings 20 and 21.Wafer 22 to be implanted is mounted on a holder in the housing 19, andthe whole target region including housing 19 and wafer holder is held atground potential. The housing 18 is then biased as required relative tothe target housing 19 to provide the required post accelerationpotential to accelerate the mass selected ions to the requiredimplantation energy.

Immediately before the accelerated beam impinges upon the wafer 22, aplasma gun 24 floods the beam and the wafer with low energy electrons toneutralise any charge accumulation on the surface of the wafer due toimplanted ions.

It will be appreciated that the entire beamline is maintained at verylow pressure. Turbo pumps 25 and 26 are provided to evacuate the ionsource and the mass selection region respectively. A further cryogenicpump 27 maintains the pressure in the target region as low as possibleto minimise contamination. Although the described apparatus is capableof further accelerating the ions after mass selection by as much as 160keV for implantation (or 190 keV for high energy implantation), theapparatus can also operate with lower implantation energies. Indeed, bybiasing the beamline region (11, 15 & 18) in the opposite direction, themass selected beam can be decelerated to below the ion source extractionenergy.

The processing speed for wafers exposed to the beam of ions forimplantation is dependent amongst other things on the beam currentdensity of required ions impinging upon the wafer. Especially for lowimplantation energy applications, there are difficulties in maintainingthe beam current of ions being implanted at satisfactory levels.

Referring now to FIGS. 2 to 4, there is shown an ion source according tothe present invention for use with the implantation apparatus of FIG. 1which can increase the proportion of the desired ions in the beamcurrent extracted from the source, so that the residual beam current ofthe desired ions implanted in the wafer can also be increased. Theassociated extraction electrodes are omitted for clarity.

The ion source indicated generally at 10 (see also FIG. 1) comprises anionisation chamber 32 which has a front plate electrode or top 102, base34 defining a floor 36, side walls 38, 40 and end walls 42, 44, all ofwhich are rectangular and form a cuboid structure collectively.Obviously other shapes may be used. The use of “top”, “bottom”, “base”,“side” and “end” are used for convenience, and do not denote a desiredor necessary orientation of the ion source in use. Within the chamber 32is provided a liner 33 which may be formed as a single unitary box-likestructure providing panels fitting in sliding relationship with theinterior surfaces of the walls 38, 40, 42, 44 of the chamber 32 or maybe formed as a plurality of liner panels 39, 41, 43, 45 which can beslotted into place, abutting those interior surfaces andinter-supporting as shown in FIG. 2. The liner panels are supported on abase liner panel 35 of the liner 33 which base liner panel is supportedin spaced relationship from the floor 36 of the chamber to define acavity 51 for the purpose hereinafter described when the source ispositioned in space as shown in FIG. 4.

The end liner panels 43, 45 are each recessed to accommodate endportions of the side liner panels 39, 41. The four liner panels 39, 41,43, 45 are seated on the base liner panel 35 as shown in FIG. 2.

In addition to the provision of a liner within the ionisation chamber,or as a preferred embodiment of the present invention, the ionisationchamber can be enclosed, wholly or partially, by a heat shield in orderto retain heat within the chamber and thereby improve the efficiency ofthe ion source. Retention of heat together with the continued plasmageneration causes an increase in temperature within the chamber, therebyincreasing the effectiveness of the dissociation and ionisation of thesource material, such as boron trifluoride (BF₃), to produce more boronions.

Such a heat shield can be created by coating the exterior surfaces ofthe chamber with heat-reflective and/or heat-insulating material. Foroptimum results, where multiple coatings are applied, one or more heatreflective coatings or one or more heat-insulating coatings may beapplied to the exterior surfaces, or these may be applied alternatelyuntil the desired level of heat shielding has been built up.

In addition, or as a preferred alternative to the provision of coatings,heat shielding can also be provided by plates of heat-reflective and/orheat insulating material to enclose all or part of the chamber. The heatshield can be preformed of box-like structure to enclose the chamber.

A preferred embodiment of ionisation chamber having a heat shieldprovided thereon is shown in FIGS. 6 and 7. The same reference numeralsare used in these Figures as are used in FIGS. 2 and 4, whereappropriate, to identify like parts of the chamber as are illustratedtherein.

The ionisation chamber 32 has a front plate 102 and is at the samepotential as the arc chamber and is surrounded by a heat shieldgenerally indicated at 160. The heat shield 160 comprises side shieldmember 162 and end shield members 164. The shield members are held inposition to surround the side and end walls 38, 40 and 42, 44respectively of the chamber 32 (40, 44 not shown in FIGS. 6 and 7) andare spaced therefrom. The means for holding the members 162, 164 inposition comprise four coupling members in the form of corner pillars170 each of which has a base portion 172 and an upright pillar portion174.

Each shield member 162, 164 comprises three plates 176 ofheat-reflective material (shown in FIG. 7). Each plate is formed ofheat-reflective material which may be stainless steel, typically ofthickness in the range of 0.25-100 mm thickness though other thicknessesmay be selected according to requirements. The three plates of eachshield member are mounted in spaced-apart relationship so that a gapexists between the centre plate and each of the other two plates. Thus,in the operating environment of the ionisation chamber 32 where it islocated within the vacuum environment of an ion implanter, each of thethree plates of each member 162, 164 reflects radiated heat which mayemanate from the walls of the ionisation chamber. A base shield member177 (similarly constructed to shield members 162) is also mountedadjacent but spaced from the base 34 of the ionisation chamber, in asimilar manner to that hereinafter described with reference to the sideand end members 162, 164.

As shown in FIG. 7, each pillar portion 174 of each corner pillar 170comprises a hollow columnar portion 178 and two wing portions 180, 182,all of which are orthogonal to the base portion 172. The wing portions180, 182 are at right angles to one another when viewed in plan. Eachwing portion 180, 182 of each corner pillar 170 has three verticalspaced-apart, parallel slots 184 formed therein, each slot being of awidth such that it can accommodate a vertical end portion of one of theplates 176. The plates may be frictionally fitted within theirrespective slots or may fit more loosely therein provided that betweenany two adjacent pillars 170, they are maintained in spaced-apartrelationship. Each wing portion 180, 182 has two vertically-spacedthreaded apertures (not shown) which, when the plates are marked intheir respective slots, are aligned with corresponding apertures in thevertical end portions of the plates so that bolts may pass therethroughto secure the plates in position.

The end walls 42, 44 have the cathode unit 46 and the second electrode(“counter cathode”) 48 extending therethrough and to accommodate these,the end heat shield members 164 are formed in two parts, as shown inFIG. 6 where both parts extend across the end wall 42 up to but not toan extent that they would touch the mounting for the cathode unit 46 orthe unit itself. The plates 176 of these heat shield members 164 areheld in spaced-apart relationship at their vertical end portionsadjacent the cathode unit mounting by vertical bars 188 which are formedwith spaced parallel vertical slots into which the plates can be fittedand secured by bolts in the same manner as in the wing portions 180, 182of the corner pillars 170.

Each shield member 162, is capped by a length of rolled over shieldingto avoid electrical breakdown from the edges of the individual plates.

Though, in the embodiment of FIGS. 6 and 7, the shield members aredisclosed as comprising three spaced-apart plates, it is to be clearlyunderstood that, depending upon the environment and operating conditionsin which the ionisation chamber is used, and the temperatures attainedwithin the chamber, it is foreseen that less than three or more thanthree plates may be employed in each shield member 162, 164. Toaccommodate more than three plates, each wing portion 180, 182, may beprovided with more than three slots, and the number and spacing of theplates may be adjusted according to requirements, as may the thicknessof the individual plates.

We have found that, when generating boron ions within the chamber from aboron fluoride source material, placing shielding around the ionisationchamber produced a significant effect upon the yield of boron ions in a2 KeV boron current. Provision of a single-layer heat shield ofheat-reflective material round the chamber produced a current increaseof about 10%, while augmenting of the heat shield by using atriple-layer construction resulted in a >15% increase in beam current.In both cases, the heat reflective material was polished stainlesssteel.

As shown in FIGS. 2 to 4, a gas supply for providing an ionisable gas,e.g. BF₃, to the ionisation chamber, as the source of ions to bedelivered ultimately to the wafer 22, is coupled to the ionisationchamber 32 through tie base 34. A connection to the gas supply isillustrated in FIGS. 2 and 4 and comprises a manifold 52 seated in anaperture 54 therefore provided in the base 34. The manifold 52 providesan inlet 53 for gaseous source material and is sealed in position by anO-ring (not shown) which is located in an annular seat 56. In use themanifold 52 couples to a gas supply via a gas supply conduit 58 shown inFIG. 1.

In addition to BF₃ as a gaseous source material, arsine and phosphinemay also be used, for example, if arsenic ions or phosphorous ions arerequired. Other source materials include GeF₄ and SiF₄ where Ge and Siions are required. Of course, other gaseous source materials, such andH₂ CO₂ and N₂ may also be used. Furthermore, solid source materials maybe employed in the ion source in powder or particulate form or in theform of a solid body. For such purpose the ionization chamber is notrequired to have an inlet for admitting source gas but may be connectedto a vaporizer.

Mounted within the chamber in close spaced relationship to the basepanel 35 is a screen provided by a rectangular plate 60 which is made ofmetal or metal laminate or is clad with metal foil to provide anelectrode which, in use, may be biased relative to the chamber.

The screen is made of a material (e.g. tungsten plate) having poorthermal conductivity (for tungsten 174 Wm⁻¹C⁻¹) so that in the presenceof a plasma, it is heated due to bombardment by the various plasmaconstituents. The screen is mounted within the chamber in electricalisolation from the chamber body and the electrodes extending into thechamber. With a positive bias being applied to the screen we haveobserved a significant increase in both argon and boron beam currentswhen argon and boron trifluoride are supplied to the chamber. It isbelieved that the presence of the heated, positively-biased screenimproves the breakdown of the source material to increase the yield ofdesired ions. This can be attributed to a number of causes, namely thatthe plasma itself is modified due to the electrostatic field associatedwith the positively-biased screen, modification of the electron energydistribution within the plasma, and the heated surface of the screenitself, leading to the improved breakdown of the source material.

Of course, the screen may itself be formed of a material capable ofaugmenting the ion beam current or it may be formed of a material whichis impregnated and/or coated with it.

The plate 60 is formed, on the face of the plate facing the base panel35, with two bosses 62, 64, one adjacent each end of the plate, andhaving cylindrical recesses 66, 68 respectively formed therein. Thebosses 62, 64 seat within apertures 70, 72 respectively formed in thebase panel 35. The apertures 70, 72 are of larger diameter than thebosses 62, 64 so that neither boss is in contact with the base panel 35.

Apertures 74, 76, corresponding to the apertures 70, 72 respectively,are formed in the base 34 and through the aligned apertures 70, 74extends a conductive cylindrical pin 77 providing a connector to a powersupply 75 to provide a potential bias to the plate 60. The pin 77 ismounted in a bush 78 having an internal bore 80 of greater internaldiameter than the diameter of the pin 77. The bush has a reduceddiameter portion 82 which seats within the aperture 74. The pin 77 isheld within the bore 80 by insulating spacers 84, 86 and has a headportion 88 which fits within the recess in the boss 62 of the plate 60.

A cylindrical support pin 90 extends through aperture 76, mounted in abush 92 having an internal bore 94 of greater internal diameter thanthat of the pin 90. The pin 90 is held within the bore 94 by spacers 96,98 and has a stepped crank portion 100 providing a boss 101 whichextends through the aperture 72 into the recess in the boss 64 of theplate 60. The two pins 77 and 90 support the plate 60 in spacedrelationship to the base liner panel 35.

The relative spacing of the base 34, the base liner panel 35, and theelectrode plate 60, permit reactant gas to flow though the manifold 52into the cavity 51 and through cutouts in the base plate 35 and aroundthe edges of the plate 60 to disperse into the interior of the chamber32.

In FIG. 4 the ionisation chamber is shown as having the top plate 102attached to the ionisation chamber 32. The plate 102 is the firstelectrode of the extraction system, though it is at the same potentialas the chamber. A liner panel may be secured to the interior surface ofthe face plate electrode though this is optional.

Extending through the plate 102, there is the outlet 13 in the form anelongate slot through which ions can exit from the ionisation chamber.An extraction electrode 14 a is maintained at a potential such as todraw ions from the plasma created inside the chamber when the chamber isoperating.

The front plate 102 which may be formed from more than one piece ofmaterial is in the shape of a rectangular plate which is formed ofgraphite. The graphite can be either impregnated and/or coated with amaterial capable or providing the same ions as are provided by theplasma within the chamber. Thus, for example, if a source gas such asboron trifluoride is introduced into the chamber for the purpose ofproviding a supply of boron ions, then the electrode is eitherimpregnated or coated with boron, boron nitride or boron carbide so thatas the electrode is bombarded and heated during operation of the ionsource, boron ions are emitted and augment the ion beam emanating fromthe ionisation chamber.

Of course, the plate 102 may be formed of other materials andimpregnated or coated with further materials capable of augmenting orcomplementing ions exiting through the outlet 13. As an alternative, inorder to augment an ion beam, the plate may simply be formed of anion-providing material. Thus, for example, if the source material in theionisation chamber provides, say, aluminium ions, then the plate may beformed of pure aluminium. As a further alternative, the liner panel maybe coated or impregnated with ion-providing material.

The ion source shown in FIGS. 2 to 4 also comprises an electrodestructure comprising a prior art cathode unit 46 and an opposed countercathode 48 mounted in opposed end walls 42, 44 of the chamber 10. Thecounter cathode 48 is mounted on an electrically-conductive post 124which extends through the thickness of, and in electrical isolationfrom, the liner panel 45 and the end wall 44 of the chamber and isconnectable to a negatively-biased power supply (not shown) at the samepotential as the cathode unit 46. It can also be run at a differentpotential to that of the cathode.

The cathode unit 46 (FIGS. 5A to 5C) according to the present inventioncomprises a cathode tube 128 which is of cylindrical shape. The externaldiameter of a typical cathode tube is about 10-20 mm with length about20-40 mm, the thickness of the tube wall being approximately 0.5-3 mm.The tube is typically formed of tungsten or molybdenum and may befinished by grit blasting with white fused alumina abrasive or in someother manner.

The cathode tube 128 carries a cathode button 130 (shown in FIG. 5C)which forms a press fit on the end of the tube and is positioned insidethe chamber 32 when the cathode unit is mounted in position as shown inFIGS. 2 to 4.

The cathode button 130 is seated at the end of the tube and has acylindrical seating portion 132 which can be pressed into the end of thecathode tube 128. The cathode button is also made of tungsten ormolybdenum, but could of course be made from any suitable material usedin the art of electron emission and may be welded to the tube.

Positioned within the tube 128 is a filament 118 of well known designand made of tungsten. The filament 118 is placed inside the tube closeto the cathode button 130 and is connected to a heater power supply (notshown). When the ion source is in use, heating of the filament causesemission of electrons therefrom and these electrons impact the cathodebutton and cause emission of electrons therefrom into the ionisationchamber interior. A potential is applied between the filament andcathode to enhance the heating from electron impact.

The filament 118 is connected by leads (not shown) in conventionalmanner to a power supply for heating the filament.

In FIGS. 2 and 4 the tube 128 has apertures 138. The purpose of theapertures 138 in the wall of the tube is to permit the filament to bereadily inserted inside the tube and to be readily removed therefromwhen it is exhausted.

Spaced along the length of the tube in the embodiment of FIGS. 5A-5C,are formed slits 140. In FIGS. 5A-5C, eight slits 140 are shown,arranged as two sets 140 a, 140 b of four slits each, each set beingarranged in a plane normal to the axis of the cathode tube 128 andspaced equidistantly about the circumference of the tube, leaving abridge portion 142 of the tube between adjacent slits, each bridgeportion having a circumferential width of about a few mm (e.g. 2 mm).Each slit 140 has an axial dimension (i.e. width) of about 0.2-0.8 mm,although these may of course be of other dimensions. We have found thatprovision of two sets of slits significantly reduces heat conductionalong the tube when the cathode is in use and becomes heated, both bythe emission of electrons from the filament, and by bombardment fromions in the plasma contained by the arc chamber, during operation of theion source of which the cathode unit forms part.

Although the cathode tube shown in FIGS. 5A to 5C has eight slits, itwill be appreciated that the effect of reduction of conduction of heatalong the cathode tube can be achieved with single slit configurationsor with more slits in various arrangements. Ultimately, there can beenough slits as to create a mesh so that the tube itself can have amesh-like appearance. It will also be appreciated that the slits mayextend diagonally around the tube.

Thus, either an increased ion current can be achieved or complimentarilythe same ion beam current can be achieved as before but with a lowerrate of gas flow into the chamber. Furthermore, with the tube being lessopen due to the absence of the large cut-out section provided byapertures 138, gas flow out of the arc chamber and also the possibilityof a plasma developing in the tube is reduced.

The cathode tube 128 extends through an aperture 150 in the end wall 42which is of a diameter such that the tube is not in contact with the endwall and is thus insulated therefrom. As previously stated, in use thecathode tube 128 may be connected to the same negative supply as thecounter cathode 48, such that electrons which are discharged by thecathode unit 46 into the ionisation chamber are repelled by the countercathode 48, and thus reciprocate between the two electrodes 46, 48.Mounted within or about the housing 11 and surrounding the ionisationchamber 32 is an arrangement of electromagnets which create a magneticfield within the chamber to entrain the electrons to move in fixed pathsbetween the two electrodes 46, 48 to interact with the source materialsupplied via the gas supply conduit 53 to produce ions within thechamber. The post 124 on which the counter electrode 48 is mountedextends through an aperture 152 in end wall 44.

It will be appreciated from the foregoing description that the variousaspects and features of the invention can be implemented independentlyof one another in an ion source according to the present invention andthat these various aspects and features of the invention are capable ofcombination within an ion source, in accordance with the presentinvention.

1. An ion source comprising an ionisation chamber within which a plasmacan be generated, the chamber having an outlet through which ions canexit from the ionisation chamber, electrodes in the ionisation chamberfor establishing and maintaining a plasma within the chamber when apower supply is provided thereto, and a heat shield enclosing at least apart of the ionization chamber to retain heat within the chamber whenthe ion source is functioning, wherein the heat shield comprises aplurality of heat shield members which extend around the ionizationchamber in spaced relationship thereto and a plurality of couplingmembers for connecting the heat shield members to form the heat shieldaround the ionization chamber, and wherein the ionization chamber is ofcuboid shape and has side and end walls and the heat shield members arearranged in spaced relationship to said side and end walls, and whereinthe coupling members are mounted adjacent intersections of side and endwalls of the ionization chamber and each coupling member comprises apillar portion extending parallel to and spaced from its adjacentintersection and wing portions extending in the general direction of theadjacent wall, the wing portions providing said slots to accommodatetherebetween the spaced parallel plates of the associated heat shieldmember.
 2. An ion source according to claim 1 wherein each heat shieldmember comprises a reflective plate mounted between adjacent couplingmembers, the heat shield members being positioned to provide for accessto said electrodes.
 3. An ion source according to claim 2 wherein eachcoupling member is provided with slots to accommodate edge portions ofadjacent plates to create said heat shield around said chamber.
 4. Anion source according to claim 1 wherein each heat shield membercomprises a plurality of spaced parallel plates which are held inrelative spaced parallel relationship by said coupling members.
 5. Anion source according to claim 4 wherein each coupling member is formedto define first and second pluralities of slots, the slots of eachplurality being arranged in spaced apart parallel relationship, and thefirst and second pluralities of slots being orthogonal to one another tomount at least two plates between adjacent coupling members so that theplates are in spaced relationship to one another and to the chamber. 6.An ion source according to claim 4 wherein each plate is formed ofstainless steel.
 7. An ion source according to claim 1 whereinfastenings are provided for securing the plates in situ once the endportions thereof have been located between the wall portions of the wingportions of the associated coupling members.
 8. An ion source accordingto claim 4 wherein a base heat shield is provided in spaced relationshipto the base of the ionisation chamber, the base heat shield beingsecured to base portions of the coupling members.
 9. An ion sourceaccording to claim 1 wherein the heat shield of the ionisation chamberis provided by an external coating of a material selected from the groupconsisting of heat-reflective material and heat insulating material. 10.An ion source according to claim 1 wherein the heat shield is providedin the form of a hollow multipart body into which the ionisation chambercan be fitted.
 11. An ion source according to claim 1 and furthercomprising a screen provided at an interior wall surface of theionisation chamber in opposed relationship to the outlet of theionisation chamber through which ions can be directed, the screen beingbiased with respect to the chamber to assist at least one of ejection ofions therefrom and the breakdown of molecular species within thechamber.
 12. An ion source according to claim 1 wherein one of theelectrodes comprises a cathode unit having a cathode tube extending intothe ionisation chamber, a cathode button mounted at the end of thecathode tube and a heating element positioned in the cathode forconnection with a power supply for negatively biasing the heatingelement relative to the cathode button to effect emission of electronsinto the ionisation chamber, the tube having a length and having atleast one slit with a direction transverse to the length such as torestrict heat conduction along the cathode tube.
 13. An ion sourceaccording to claim 1 and further comprising a front face plate for theionisation chamber for use in extracting ions from the ionisationchamber for supply to said ion implantation apparatus for implantationinto a substrate, the front face plate providing the outlet of theionisation chamber, through which ions can exit the ionisation chamber,the face plate being formed of a material which will not contaminate theflow of ions, and bearing a material selected from an element, and acompound thereof, the ions of which element are the same specie as thoseto be implanted into the substrate from the source thereof.
 14. An ionsource comprising an ionization chamber within which a plasma can begenerated, the chamber having an outlet through which ions can exit fromthe ionization chamber, electrodes in the ionization chamber forestablishing and maintaining a plasma within the chamber when a powersupply is provided thereto, and a heat shield enclosing at least a partof the ionization chamber to retain heat within the chamber when the ionsource is functioning wherein the heat shield comprises a plurality ofheat shield members which extend around the ionization chamber in spacedrelationship thereto and a plurality of coupling members for connectingthe heat shield members to form the heat shield around the ionizationchamber, and wherein the ionization chamber is of cuboid shape and hasside and end walls and the heat shield members are arranged in spacedrelation ship to said side and end walls, wherein two electrodes areprovided in the ionisation chamber, a first electrode extending throughone end wall of the ionisation chamber and the second electrodeextending through the other end wall thereof, and each heat shieldmember provided adjacent an end wall being formed in at least two partsso as to extend from its coupling member adjacent the intersection ofthat end wall with its associated side wall to a location adjacent butspaced from the adjacent electrode.
 15. An ion source comprising anionisation chamber within which a plasma can be generated, an outletthrough which ions can exit the chamber, electrodes in the ionisationchamber for establishing and maintaining a plasma within the chamberwhen a power supply is provided to the electrodes, a screen provided atan interior wall of the ionisation chamber, electrically insulatedtherefrom, the screen being heated during operation of the ion source toassist at least one of ejection of ions therefrom through said outletand the break down of molecular species within the arc chamber, and apower supply connected to bias the screen positively relative to thechamber.
 16. An ion source according to claim 15 wherein said screen ismounted in spaced relationship to said interior wall.
 17. An ion sourceaccording to claim 15 wherein said screen is mounted on said interiorwall.
 18. An ion source according to claim 15 wherein said screen isformed of low thermal conductivity material.
 19. An ion source accordingto claim 18 wherein said material is metallic.
 20. An ion sourceaccording to claim 19 wherein said material is selected from the groupconsisting of tungsten, molybdenum and compounds and alloys thereof. 21.An ion source according to claim 19 wherein said material is selectedfrom the group consisting of an element, compound thereof and alloythereof which is capable of ionisation when the ion source is inoperation.
 22. An ion source according to claim 15 wherein said screenis provided by a rectangular plate.
 23. An ion source according to claim22 wherein the plate is mounted on support pins which extend through thewall of the ionisation chamber.
 24. An ion source according to claim 23wherein the plate is supported on said support pins in spacedrelationship to the wall of the ionisation chamber.
 25. An ion sourceaccording to claim 24, wherein at least one of said support pins iselectrically conductive for coupling the screen to a bias supply forbiasing the screen with respect to the chamber.
 26. An ion sourceaccording to claim 15 wherein an inlet port is provided in said chamberthrough which reactant gas can be introduced into the ionizationchamber, and the screen overlies said inlet port to cause reactant gasto disperse around it when flowing into the ionisation chamber.
 27. Anion source according to claim 15 wherein the cathode unit comprising acathode tube extending into the ionisation chamber, a cathode buttonmounted at the end of the cathode tube and a heating element positionedin the cathode tube for connection with a power supply negativelybiasing the filament relative to the cathode button to emit electrons,thereby to heat the cathode button to cause the cathode button to emitelectrons into the ionisation chamber, the tube being having a lengthand having at least one slit with a component of which is transverse tothe length such as to restrict heat conduction along the cathode tube.28. An ion source according to claim 15 and further comprising a frontface plate for the ionisation chamber and providing a chamber outlet,the plate being formed of a material which will not contaminate the flowof ions, so that ions from the chamber may pass therethrough, andbearing a material selected from an element, and a compound thereof, theions of which element are the same specie as those to be implanted intothe substrate from the source thereof.
 29. An ion source for an ionimplanter, comprising an ionisation chamber having opposed wallportions, an outlet through which ions can exit from the chamber, and anindirectly heated cathode, said indirectly heated cathode comprising acathode tube extending through one of said opposed wail portions, acathode button mounted at an end of the cathode tube within theionization chamber, and a heater element positioned in spatial relationin the cathode tube for connection with a power supply negativelybiasing the element relative to the cathode button so that in useelectrons emitted by the heated element are accelerated to the button toheat the cathode button by electron impact, thereby to cause the cathodebutton in use to emit electrons into the ionisation chamber, whereinsaid cathode tube is cylindrical having an axis and a uniform externaldiameter of between 10 and 20 mm, and has at least one slit with acomponent transverse to the axis such as to restrict heat conductionalong the cathode tube.
 30. An ion source according to claim 29 andcomprising at least two slits spaced along the length of the cathodetube.
 31. An ion source according to claim 29 and comprising at leasttwo slits spaced around the cathode tube.
 32. An ion source according toclaim 31 wherein the slits are uniformly spaced around the tube.
 33. Anion source according to claim 31 wherein each slit lies in a respectiveplane which is normal to the direction of the length of the cathodetube.
 34. An ion source according to claim 29 wherein a plurality ofsaid slits is arranged in a discontinuous ring around the tube.
 35. Anion source according to claim 34 wherein the ring is formed by betweentwo and eight evenly-spaced slits.
 36. An ion source according to claim32 wherein the slits are of at least substantially equal length.
 37. Anion source according to claim 34 wherein a neck portion of the cathodetube is retained between adjacent slits of a ring of slits.
 38. An ionsource according to claim 35 wherein the slits are formed in the cathodetube sufficiently closely and in such spaced relationship to one anotherthat the cathode tube defines a tubular mesh.
 39. An ion sourceaccording to claim 29 wherein the cathode tube and the cathode buttonare formed of a material selected from the group consisting of tungstenand molybdenum.
 40. An ion source according to claim 29 wherein thecathode button is shaped to provide a press fit with and at the end ofthe cathode tube.
 41. An ion source according to claim 29 wherein thecathode tube and the cathode button are integrally formed as a singleone piece unit.
 42. An ion source according to claim 29 wherein thecathode tube is of circular cross-section.
 43. An ion source accordingto claim 29 wherein an aperture is formed in the body of the cathodetube for locating a heating element within the cathode tube adjacent thecathode button and for withdrawing a spent cathode button.